The present invention relates to improvements in or relating to spread spectrum signalling schemes and has particular but not exclusive application to the transmission of low power spread spectrum signals from low power transmitters which may be included in message receiving apparatus such as selective call receivers, for example digital pagers or hand held digital signal processing apparatus.
U.S. Pat. No. 4,882,579 discloses an acknowledge back (ack-back) paging system in which a central station transmits a sequence of addresses of a group of M different ack-back pagers for whom there are paging signals and after an interval this is followed by a sequence of messages to the group of ack-back pagers. The order of the messages is related to the order of the addresses in the transmitted sequence in a predetermined manner. The users of the group of addressed ack-back pagers indicate a response, for example by pressing a button, to their respective pagers thus providing ack-back data. The pagers in the group of addressed ack-back pagers then simultaneously transmit back to the central station their ack-back signals using different pseudo-random codes, a different pseudo-random code being allocated to each of the pagers in the group. In order to permit simultaneous transmissions the pagers must send the transmitted acknowledgement signal with extreme accuracy in frequency. In order to facilitate accurate frequency tuning the central station transmits a burst of reference carrier in a time interval between the transmissions of the sequence of addresses and the sequence of messages and the pagers have to include frequency control circuitry which permits such accuracy in frequency tuning. Additionally power control techniques are applied by the pagers to regulate transmitter output power.